1. Field of the Invention
The present invention relates to a process of fabricating a magnetic disk memory device, and in particular, to a method of detecting defects in the surfaces of a magnetic disk in a sequence of fabrication processes and rendering a defective disk surface unavailable in a user environment.
2. Description of the Related Art
A hard disk drive widely used as an auxiliary memory device for a computer system is a major magnetic disk memory device. In the hard disk drive, data is recorded in tracks concentrically arranged on a magnetic disk. A head or a data transducer accesses these tracks to read data from, write data to erase data from, the disk.
In general, it is impossible to fabricate a perfect defect-free disk in view of its fabrication process, and even possible fabrication of perfect defect-free disks leads to a low product yield and a high production cost. Thus, a disk manufacturer and a hard disk drive manufacturer allow disks to be defective to a certain extent. New defects may be produced in a disk during a fabrication process or in a field after it is completely fabricated because it is fabricated of a material vulnerable to shocks.
Meanwhile, the trend of hard disk drives goes toward increasing the recording density of a disk to increase the storage capacity of the disk. Along with this, various parts such as a head have been developed toward high precision. As a result, micro defects, which caused negligibly small and correctable errors before, now generate data errors in so many bits that they cannot be corrected.
A so-called bum-in process is performed to detect such defects beforehand and to determine a corresponding disk to be defective or not according to the number of defects. The bum-in test is implemented on a rack in a bum-in room without an additional test system by a firmware, consuming generally 8-16 hours, longer than any other step in the hard disk drive fabrication process, so that defects are detected from a disk surface beforehand to avoid a defective portion when the drive is used and thus contribute to consumer's reliable use of the hard disk drive.
A defect sector detecting test is performed during the burn-in step in the hard disk drive fabricating process, wherein a microprocessor for controlling the whole operations of a hard disk drive performs a write/read test on the entire areas of a disk surface, applying a stress on a read/write channel by causing a magnetic head to move off track or changing a read/write channel parameter, in order to find a defect sector from the disk surface. In the case of an off track stress, a head does not follow the center of the track, and a read/write operation is abnormally executed. In the case of the above abnormal read/write operation, because an error due to a micro defect is amplified, the defects of a disk can be easily detected. During the write/read test, a sector having a write/read error (i.e., a defect) is detected. The address of the defect sector is written in a defect list of a predetermined area, that is, a maintenance area on the disk surface and is not accessed later in a user environment, to thereby provide a defect-free hard disk drive to a user.
However, in the above conventional test for detecting a defective sector, there is difficulty in accurately detecting a defect in a disk with respect to pattern variations and in determining that a signal having no margin is defective because a write/read test pattern designated in a burn-in script file is written/read while a scrambling function is enabled. The scrambling function refers to avoiding writing symbols of the same codeword in an identical group and writing data randomly according to patterns given from different codewords.
In the case of a micro defect, for example, when data is written in a defective portion in a high-frequency pattern or a low-frequency pattern, magnetic components are not accurately recorded due to the defect. Thus, when reading the data, the amplitude of a data bit is smaller than that of a bit normally written or the data bit is damaged, leading to a read error. On the other hand, when the data is written in the defective portion in the low-frequency pattern, the data bit may not be damaged if the defective portion is small. In this case, no error is generated during a write/read operation, thereby impeding an accurate test.
Techniques for preventing the use of bad disks or bad sectors of a disk are discussed in the following patents incorporated by reference: U.S. Pat. No. 5,784,216 to Dan Zaharris entitled Method And Apparatus For Recording Defective Track Identification Information In A Disk Drive; U.S. Pat. No. 5,737,519 to Ghassan Maurice Abelnour et al. entitled System And Method For Early Disk Drive Fault Detection; U.S. Pat. No. 5,047,874 to Ruben Yomtoubian entitled Technique For Certifying Disk Recording Surface; U.S. Pat. No. 4,924,331 to Don M. Robinson et al. entitled Method For Mapping Around Defective Sectors In A Disc Drive; and U.S. Pat. No. 4,214,280 to Martin O. Halfhill et al. entitled Method And Apparatus For Recording Data Without Recording On Defective Areas Of A Data Recording Medium.